A recent meta-analysis of 2 randomized, controlled field trials conducted in Santiago, Chile, with the licensed live oral typhoid vaccine Ty21a showed that vaccination not only conferred protection against S. Typhi disease, but also against paratyphoid B fever. The overall goal of this application is to study the immunological mechanisms that could mediate this cross-protection. The proposed studies will focus on investigating whether the array of cell-mediated immunity (CMI) and Ab responses elicited by immunization of subjects with the licensed Ty21 a typhoid vaccine, as well as novel attenuated S. Typhi vaccine candidates, which are likely to be involved in protection, cross-react with S. Paratyphi A and B antigens. Specifically, we propose to address the following Specific Aims: Aim 1. Test the hypothesis that a defined set of CMI responses in circulation play a key role in cross-protection between S. Typhi strain Ty21a and S. Paratyphi B infection in humans. To this end, we will use cells from subjects immunized with Ty21a to evaluate whether there is cross-reactivity between CMI responses (e.g., cytokine production, proliferative responses, cytotoxic activity) to S. Typhi, S. Paratyphi A and S. Paratyphi B. We hypothesize that crossreactivity will be chiefly observed between S. Typhi and S. Paratyphi B, with minor, if any, cross-reactivity with S. Paratyphi A antigens, Aim 2. Evaluate the hypothesis that the CMI responses observed in Ty21a vaccinees that show cross-reactivity between S. Typhi and S. Paratyphi B antigens (Aim 1) are also elicited by oral immunization of subjects with novel attenuated S. Typhi candidate vaccine strains CVD 908-htrA and CVD 909. These studies will assess the likelihood that immunization with these novel attenuated S. Typhi candidate vaccine strains will result in a broad-spectrum S. Typhi and S. Paratyphi B (and perhaps S. Paratyphi A) vaccine against enteric fevers, and Aim 3. Evaluate the hypothesis that immunization of subjects with Ty21a and novel attenuated S. Typhi candidate vaccine strains CVD 908-htrA and CVD 909 elicits the appearance in serum ofAb that cross-react to Salmonella common antigens present in S. Typhi and S. Paratyphi B (e.g., flagella, OmpC, GroEL, O-polysaccharide epitope 12). These studies will include the measurement of circulating antibody secreting cells (ASC), memory B cells (BM) and Ab directed to common Salmonella antigens, as well as IgA-mediated ADCC. The presence of these cross-reactive Ab, ASC and BM cells will suggest that these humoral responses might play an active role in cross-protection.